This type of virus uses any one of a variety of techniques to minimize detection of its activity.
In order to spread widely, a virus must attempt to avoid detection. To minimize the probability of its being discovered a virus could use any number of different techniques. It might, for example, only infect every 20th time a file is executed; it might only infect files whose lengths are within narrowly defined ranges or whose names begin with letters in a certain range of the alphabet. There are many other possibilities.
A virus which uses such techniques is termed a sparse infector.
A wide variety of techniques can be used to help a virus avoid detection of its activity.